Well this is really disheartening.

Ater rebuilding my motor (used flat-top hypereutectic pistons, Sportsman
II 64 cc heads, stock intake and q-jet, new stock waterpump and 180 t-stat,
ect ect), even installed a new three-row modine stock-type radiator to replace
my sad origional

I babied it and only drove it the ten miles to school and ten miles back
each day until I finally got the motor to about 500 miles. So I figure I'll
go for a nice cruise since the motor had had time to cut the rings.

The temps skyrocketed at highway speeds after about 25 minutes of driving.
They got up to about 240 or so. But they are fine at idle or near idle!
I even sat in horrible rush hour traffic (took 2 hours to go 8 miles), and it
never went over 225.

At first I just had a 14 inch electronic cooling fan on it. Now I have the electric
and the origional clutch fan (no shroud) pulling together, and its still doing the
same thing!

I've got a 16 inch fan on another car that doesn't need it (the other car (V6
nissan BTW) runs cool except sitting still, and the fan rarely turns on).
I'm considering swapping the fans to see if it helps the vette any.

The local mechanic suggested that I may be running too lean with my stock
l42 quadrajet being as my new motor puts out a lot more HP. Could this
be the case?

 

Running too lean could easily be a cause of your problem. There are a few other things to check too.

Do you have all of the seals around the radiator to force the air through the radiator, rather than letting it bypass and go straight out the engine compartment?

Do you have a chin spoiler on your vette?

Does your lower radiator hose have a spring in it to avoid collapsing? They've been known to collapse at high RPM's but be fine at lower RPM's.

Have you checked the coolant level since running the engine? You may have had enough in the radiator but then once you ran the engine, the coolant filled the block and the level in the radiator is low.

All of these items are necessary.

Do a search on overheating. Another guy had the same problem, see what others suggested to him.


[Modified by sharklover, 11:40 PM 3/17/2002]

 

You could be right about being lean. I tried waaay lean jets in a 3310 vacuum Holley once for gas milage purposes. It would pop through the carb when going up hill it was so lean. Also, do you have the chin spoiler thingy still attached to your car? My car ran alot cooler when I put it back on. Helped direct more air through the rad I guess.